【温故知新】c#事件event

从上一篇文章【温故知新】C#委托delegate可知，委托delegate和事件Event非常的相似，区别就是event关键字，给delegate穿上了个“马甲”。

让我们来看官方定义：

类或对象可以通过事件向其他类或对象通知发生的相关事情。 发送（或引发）事件的类称为“发行者”，接收（或处理）事件的类称为“订户”。

event 关键字用于在发行者类中声明事件。

定义非常明确，通过事件向其他类或对象通知发生的相关事情，用来实现的观察者模式。

还是通过之前的代码例子，看看声明delegate和event

        //声明一个委托类型,通知家长
        public delegate void NotifyDelegate(string msg);

        //老师被吩咐了1个委托
        //声明委托：在发现早恋时时通知家长
        private NotifyDelegate NotifyStudentLove;

        //声明事件，如果发现学生早恋！ 就要通知那些订阅了这个事件的家长。
        public event NotifyDelegate FindStudentLove;

让我们通过IL DASM来看看编译之后事件event的真正面目~

.event Delegate.Teacher/NotifyDelegate FindStudentLove
{
  .addon instance void Delegate.Teacher::add_FindStudentLove(class Delegate.Teacher/NotifyDelegate)
  .removeon instance void Delegate.Teacher::remove_FindStudentLove(class Delegate.Teacher/NotifyDelegate)
} // end of event Teacher::FindStudentLove

.method public hidebysig specialname instance void 
        add_FindStudentLove(class Delegate.Teacher/NotifyDelegate 'value') cil managed
{
  // 代码大小       48 (0x30)
  .maxstack  3
  .locals init (class Delegate.Teacher/NotifyDelegate V_0,
           class Delegate.Teacher/NotifyDelegate V_1,
           class Delegate.Teacher/NotifyDelegate V_2,
           bool V_3)
  IL_0000:  ldarg.0
  IL_0001:  ldfld      class Delegate.Teacher/NotifyDelegate Delegate.Teacher::FindStudentLove
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  stloc.1
  IL_0009:  ldloc.1
  IL_000a:  ldarg.1
  IL_000b:  call       class [mscorlib]System.Delegate [mscorlib]System.Delegate::Combine(class [mscorlib]System.Delegate,
                                                                                          class [mscorlib]System.Delegate)
  IL_0010:  castclass  Delegate.Teacher/NotifyDelegate
  IL_0015:  stloc.2
  IL_0016:  ldarg.0
  IL_0017:  ldflda     class Delegate.Teacher/NotifyDelegate Delegate.Teacher::FindStudentLove
  IL_001c:  ldloc.2
  IL_001d:  ldloc.1
  IL_001e:  call       !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<class Delegate.Teacher/NotifyDelegate>(!!0&,
                                                                                                                          !!0,
                                                                                                                          !!0)
  IL_0023:  stloc.0
  IL_0024:  ldloc.0
  IL_0025:  ldloc.1
  IL_0026:  ceq
  IL_0028:  ldc.i4.0
  IL_0029:  ceq
  IL_002b:  stloc.3
  IL_002c:  ldloc.3
  IL_002d:  brtrue.s   IL_0007
  IL_002f:  ret
} // end of method Teacher::add_FindStudentLove

.method public hidebysig specialname instance void 
        remove_FindStudentLove(class Delegate.Teacher/NotifyDelegate 'value') cil managed
{
  // 代码大小       48 (0x30)
  .maxstack  3
  .locals init (class Delegate.Teacher/NotifyDelegate V_0,
           class Delegate.Teacher/NotifyDelegate V_1,
           class Delegate.Teacher/NotifyDelegate V_2,
           bool V_3)
  IL_0000:  ldarg.0
  IL_0001:  ldfld      class Delegate.Teacher/NotifyDelegate Delegate.Teacher::FindStudentLove
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  stloc.1
  IL_0009:  ldloc.1
  IL_000a:  ldarg.1
  IL_000b:  call       class [mscorlib]System.Delegate [mscorlib]System.Delegate::Remove(class [mscorlib]System.Delegate,
                                                                                         class [mscorlib]System.Delegate)
  IL_0010:  castclass  Delegate.Teacher/NotifyDelegate
  IL_0015:  stloc.2
  IL_0016:  ldarg.0
  IL_0017:  ldflda     class Delegate.Teacher/NotifyDelegate Delegate.Teacher::FindStudentLove
  IL_001c:  ldloc.2
  IL_001d:  ldloc.1
  IL_001e:  call       !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<class Delegate.Teacher/NotifyDelegate>(!!0&,
                                                                                                                          !!0,
                                                                                                                          !!0)
  IL_0023:  stloc.0
  IL_0024:  ldloc.0
  IL_0025:  ldloc.1
  IL_0026:  ceq
  IL_0028:  ldc.i4.0
  IL_0029:  ceq
  IL_002b:  stloc.3
  IL_002c:  ldloc.3
  IL_002d:  brtrue.s   IL_0007
  IL_002f:  ret
} // end of method Teacher::remove_FindStudentLove

实际上编译器会帮你生成如下类似代码：

// 1. A PRIVATE delegate field that is initialized to null 
private EventHandler<NewMailEventArgs> NewMail = null; 
// 2. A PUBLIC add_Xxx method (where xxx is the Event name) 
// Allows objects to register interest in the event. 
[MethodImpl(MethodImplOptions.Synchronized)]
public void add_NewMail(EventHandler<NewMailEventArgs> value) { 
　　NewMail = (EventHandler<NewMailEventArgs>) 
　　Delegate.Combine(NewMail, value); 
}
// 3. A PUBLIC remove_Xxx method (where Xxx is the Event name) 
// Allows objects to unregister interest in the event. 
[MethodImpl(MethodImplOptions.Synchronized)]
public void remove_NewMail(EventHandler<NewMailEventArgs> value) { 
　　NewMail = (EventHandler<NewMailEventArgs>) 
　　Delegate.Remove(NewMail, value); 
}

---

当一个声明delegate前添加event之后，编译器为我们封装了delegate，这样，在之后的调用，就开放了+=，-=两个方法，这样极大保证了对象安全。

我们在使用c#内置事件的时候，总会发现EventHandler，EventArgs。这是因为.NET Framework为了规范，方便开发，已经为事件发布了准则。

.NET Framework 类库中的所有事件均基于 EventHandler 委托，定义如下：

public delegate void EventHandler(object sender, EventArgs e);

让我们把上一篇的代码改为符合.NET Framework事件准则

主场景：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace Delegate
{
    class Program
    {
        static void Main(string[] args)
        {
            //家长A,B
            Parent pa = new Parent();
            Parent pb = new Parent();

            //家长A,B分别委托老师发现早恋情况时通知他们
            Teacher teacher = new Teacher();
            teacher.FindStudentLove += pa.ReceiveMsg;
            teacher.FindStudentLove += pb.ReceiveMsg;

            //老师开始检查早恋情况
            teacher.CheckLove();

        }
    }
}

家长类：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Delegate
{
    public class Parent
    {
        /// <summary>
        /// 接收消息
        /// </summary>
        /// <param name="msg">通知消息</param>
        public void ReceiveMsg(object sender, StudentLoveEventArgs arg)
        {
            Console.WriteLine("家长收到通知：" + arg.Message);
        }
    }
}

老师类：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Delegate
{
    public class StudentLoveEventArgs : EventArgs
    {
        public StudentLoveEventArgs(string s)
        {
            message = s;
        }
        private string message;

        public string Message
        {
            get { return message; }
            set { message = value; }
        }
    }

    public class Teacher
    {

        //声明一个委托类型,通知家长
        public delegate void NotifyDelegate(string msg);

        //老师被吩咐了1个委托
        //声明委托：在发现早恋时时通知家长
        private NotifyDelegate NotifyStudentLove;

        //声明事件，如果发现学生早恋！ 就要通知那些订阅了这个事件的家长。
        public event EventHandler<StudentLoveEventArgs> FindStudentLove;

        //如果还想委托老师发现学生玩手机的时候通知一声，再声明一个委托即可
        private NotifyDelegate NotifyStudentPlayMobile;

        //封装委托，使其符合面向对象,event关键字就为我们自动封装了。
        public void add_NotifyStudentLove(NotifyDelegate newdelegate)
        {
            NotifyStudentLove += newdelegate;
        }

        public void CheckLove()
        {
            //某一天AB同学突然发生纠纷！被老师发现啦！
            string msg = "A同学和B同学早恋啦！！";
            //检查是否有人委托老师办事
            if (FindStudentLove != null)
            {
                //果断通知家长
                FindStudentLove(this, new StudentLoveEventArgs(msg));
            }
        }

    }
}